I think most times it was over 2 calls but that was also at a time when even on the DM/PM line you could get through in just a couple minutes. I haven't tried this recently or in the past year when call backs were several hours. Had that been the case, I probably would have done it in one call.

Sure, but making 2 calls is slightly different qualitatively than not asking at all? Of course that was before COVID-era hold times. If I tried this today I'd probably do it in a single call.

It's within the letter of the law, but not the spirit.
Sure there's evidence. If Delta intended for you to use a certificate on the initial flight, it would have made seats available. It's clearly a work around. I'm not saying don't do it, I'm saying be smart and don't screw up the current benefits by being stupid because you think you are right and entitled.
I'm 100% sure they will change the rules at some point, but we don't need to hasten that day. Every rule changes at some point. Inline with my experiences chatting and interacting with ATOBTTR, he was thoughtful in his approach regarding this issue.

I have SDC'd multiple times w/o a problem. Sounds like you ran into some improperly trained agents.

Why would they? As it relates to paid premium tickets, the SDC policy and benefit is very clearly laid out on Delta.com. If you are talking about SDC on a ticket upgraded using a certificate, it becomes less obvious. You should still be able to do it, but if you find a clueless agent, arguing with them is much less likely to produce a positive result. The main reason being the language on Delta.com does not explicitly include RUC upgraded tickets. It is implicit based on the statement that complimentary medallion upgrades are excluded. Once upon a time it was explicit in the reference materials available to agents, but they had to dig to find it. An agent sent me a copy and paste of the language, but it was in an old system that I understand they now no longer use. HUCA or just call Singapore for RUC related issues. They seemed to be trained well on this issue.

But what I'm suggesting isn't "to use a certificate on the initial flight" and is in fact to use it on the flight that has seats available.

The violation of the spirit of the rules is just something you've imagined for some reason I don't understand..

idk man that might be against the spirit of the rules, be careful or they'll take it away

and yet, this message board is littered with reports from people who have had trouble doing this

Just as a datapoint, I ran into an agent today while doing an SDC on a flight with a companion that had confirmed RUCs with tickets reissued, and this agent was a rules stickler (would not waive the $75 fee for the non-status companion to SDC, and even "checked" to verify the rules), and did not blink at all at SDCing to flights that absolutely did not have RUC upgrade inventory (I had already HUCA'd since the first agent I got was obviously new and was convinced we needed upgrade space).

Unless something has changed again, they eliminated the language prohibiting no SDC F->D1. (I want to say around when we got free DoD upgrades on domestic D1 routes)

In fact I did SDC in app from OAK-SLC-JFK paid F to OAK-LAX-JFK this summer.

Have a nonstop tomorrow, confirmed RUC at booking and ticket reissued.

Trying to SDC to an earlier flight with a connection through ATL to get in earlier, all of the earlier flights from my origin to ATL are completely sold out. Try to SDS, of course that doesn't work since the routing is changing. Try to SDC to a later flight through ATL with the connection I want so I can SDS to the earlier flight, agent won't let me claiming that I can't SDC from a non-stop to a connecting itinerary. :( HUCA, 1 hour 30 minute call back time.

Call singapore, agent won't let me SDC into F (Y is sold out on the flight). HUCA, get same answer from 2nd singapore agent.

Of course this morning all of the flights had tons of seats, I think people coming from cancelled Southwest flights are sucking up all the seats.

Got the callback from the 1.5 hour quote hold time. This guy gave me a new one, he agrees that with a confirmed RUC I can SDC into any F seat, but he's saying that since there's no Y seat available on the flights I want, the system won't let him SDC me (he says he can only SDC me into a Y seat, and THEN confirm the upgrade into F, which sounds... bogus?)

that’s stupid. If that’s really the case, they can “oversell” Y for all of 5 seconds in order to confirm you. I’d keep pushing.

It is bogus. The system has to be overridden for a lot of F SDC. While what you want is allowed it only works automatically if the same fare classes (most F fares use dual inventory) are available. Agent just needs to override.

Anyway, it worked out, was able to make an SDC this morning and currently sitting in F en route to SLC.